Some people might think I'm crazy, but if you're going to play on a modern TV, a proper video scaler is a must. And in my mind, there's really only one scaler worth buying, the xRGB mini. It's not cheap though (~$450), and you'll have to import it from Japan.
Now, you may be thinking, why the hell do I need a 500 dollar box from japan to play my old consoles? Well, there's a couple of reasons.
1. It'll standardize and convert all your inputs (rgb/scart, component, composite, svideo) into HDMI. Let's face it, HDMI isn't going anywhere soon, and analog connections on new TVs are starting to fade away. Sure, most TVs still include composite connections, but SVideo on TVs are getting harder and harder to find.
2. It'll reduce your input lag a little, especially if your TV is a horrendous internal scaler. Now, it won't do nothing to fix the inherent display lag that's evident in lcds themselves (depending on the panel technology, some lcds have much lower lag/refresh rate than others). Chances are, your TV has a poor scaler and will introduce extra frames of lag. Ever play something like super mario world, and something feels off? Like you press the jump button and the action is delayed ever so slightly? Chances are, it's the internal scaler.
3. It'll just make the picture look better. A lot of TVs introduce extra artifacts into the videos. It's just a product of...again... a bad scaler. If you have Final Fantasy 3/6, look at the opening scene in the game where you're walking around the town. There's grates on the floor. When you move vertically, everything looks fine. Move in a horizontal motion, and the grates blur. This shouldn't happen. I've seen it happen on every LCD I've ever plugged my SNES into, but it shouldn't be there. However, with my SNES plugged into the xrgb mini, this artifacting and blurring does not occur any more.
In my mind, there's three choiced: save up and buy an external scaler (there's others, but the mini just happens to be the best), live with a terrible picture on an lcd/plasma, or find a corner to stash an old CRT into.